The subject invention relates to a system for reading data from remote tags and for writing data into those tags. There are a number of prior art object and vehicle identification systems which use electronic tags containing fixed data about the particular object. These electronic tags were originally developed by the United States National Laboratory at Los Alamos, N. Mex., for the Department of Agriculture to identify livestock animals. A tag containing the animal's identification data in coded, electronically readable form, is attached to or implanted in the animal. Electronic readers are placed at feeding stations to read the tags as the animals passed nearby. These readers, also called interrogators, transmit an RF signal which, in turn, is "backscatter-modulated" with the identification data from the tag. These tags have an antenna which transmits the backscatter-modulated signal, containing the information from the tag, to the interrogator. These early systems are described in U.S. Pat. Nos. 4,075,632 and 4,360,810.
As the technology progressed, it was found that these systems for identifying remote moving objects had utility far beyond animal identification. For example, the tags may be placed in any moving or moveable objects, such as automobiles, shipping containers or railroad cars. The information in these tags can be used to keep track of the location, identity and contents of these objects. One important use of these systems is for automated toll collection on a tollway. Automobiles passing through a toll lane need not stop to deposit cash. The electronic interrogator alongside of the toll lane electronically identifies the moving car as it passes by, and triggers a computerized debiting system so that the car owner's credit card may automatically be debited for the amount of the toll. Similarly, shipping containers or railroad rolling stock can be continually monitored at ports, loading docks or along the tracks so that their location, identity and contents can be continually monitored on a central computer. U.S. Pat. Nos. 4,739,328; 4,782,345; 4,786,907; 4,816,839; 4,835,377 and 4,853,705 all describe aspects of these systems, including antennas and programmers used in these systems to identify remote objects.
In certain applications, it has become important not only to read the data contained in these electronic tags attached to the moving objects, but also to modify that data, or to "write" data into the tags while they are in motion. For example, it may be important not only to know the contents of a railroad car but also the route and time it took to reach a particular destination. Such route data, or data from which information may be calculated by a computer, may be written into the tag as it passes interrogators along its route. U.S. Pat. No. 4,390,880 describes one technique for both writing data into and reading data out of such a tag. In this patent, the interrogator at the remote receiving station first transmits to the tag, prior to being able to identify the tagged object and prior to transmitting any information-bearing signals to the tag, an "interrogation command signal". This signal tells the tag to transmit the identification data stored in the tag's memory to the transmitter. After the interrogator recognizes this identification data, it sends a "key signal" to the tag to enable a WRITE operation. The tag first must receive and recognize this key signal transmitted from the interrogator before the WRITE may take place.
This sequence requires that, prior to the remote receiving station (the interrogator) receiving any identification information back from the tag, that the tag first be "enabled" by the receipt and recognition of the transmitted interrogation signal. The tag and the interrogator must therefore be in sufficiently close proximity so that the tag first can receive and understand this interrogation signal from the transmitter, recognize it, and finally retransmit the data stored in its memory back to the interrogator. This need for the tag to be in close enough proximity to the interrogator to receive the enabling interrogation signal reduces the time window available for reading the contents of the tag and for writing data into the tag. In some applications, such as reading and writing data from and into a fast-moving train, this "handshake" delay can be sufficient to miss the tag completely.
The system of this invention considerably lessens this handshake delay by employing a tag which continually indicates its own identity, even in the absence of any interrogating command signal from the interrogator. No interrogation command signal is transmitted by the interrogator to enable reading of the tag. The tags of the subject invention use a continuously scrolling fixed code to transmit their identity. This same transmitted code also tells the interrogator whether or not the tag is in sufficiently close proximity so that the interrogator can confidently transmit and write data to the tag. Since the maximum range at which the interrogator can read the backscatter-modulated, transmitted signal from the tag is considerably greater than the maximum write range allowable for the interrogator to write data into the tag, the ability of the interrogator to read all the necessary information from the tag in advance of the tag coming into write range saves critical time. Accordingly, the technique used in this invention substantially speeds up reading and writing, greatly increasing the maximum operational range.
For example, a tag of this invention can be read as far as 80 feet away from an interrogator, but can only be written to at a maximum distance of about 20 feet. To the contrary, the tags described in U.S. Pat. No. 4,390,880 can only be read at about 6 feet and written into at about 4 feet. In the system of the subject invention, a tag may first be read and identified as it approaches a distance of about 80 feet from the interrogator. It may be written to at about 20 feet, saving the critical time that the tag is within the 20 feet write range of the interrogator exclusively for the more range-limited write operation.